Preheat the oven to 350 degrees F. Prick the sweet potatoes and yams all over with a fork and place on a baking sheet. Bake until tender, but still slightly firm, about 60 to 70 minutes. When cooked, remove and cool until safe enough to handle.
Brush the bottom of a 9- x 13-inch casserole with 1 Tbsp. of the butter. Carefully remove the skin from the sweet potatoes and yams with a small knife. Slice into 1/3- to 1/2-inch thick rounds. Lay an over-lapping row of sliced sweet potatoes in the casserole, and then lay an over-lapping row of yams. Repeat until all are used. Combine the remaining melted butter, rum, maple syrup and spices in a bowl. Drizzle over the sweet potatoes and yams. Season with salt and pepper; sprinkle with the crumbled pecan halves and cranberries, if using. (Can be made to this point several hours in advance; store in the fridge until ready to bake.) Cover and bake in 350 degrees F oven 30 minutes, or until heated through.